This invention relates to a superconductor-carbon fiber composite comprising a high strength, ultrahigh modulus, high thermal conductivity carbon fiber which is coated with a ceramic-type superconductor. More particularly, this invention relates to superconducting, super-conductor-carbon fiber composites comprised of a high strength, ultrahigh modulus, high thermal conductivity, low resistivity carbon fiber which is coated with an adhering layer of a ceramic-type superconductor such as a rare earth (R.E.), Ba, Cu, oxide-type superconductor (1-2-3,superconductor), which composite is capable of achieving significant current densities at high magnetic field strengths under superconducting conditions. The term carbon fiber as used herein includes both a carbon monofilament as well as a bundle of monofilaments (a yarn).
Recently, a number of published reports have appeared which describe superconducting ceramic-type materials composed of a combination of rare earth (e.g. yttrium) oxide, barium oxide and copper oxide which have significantly higher superconduction transition temperatures than earlier materials such as Nb/Ti alloys, niobium carbonitride and the like. Superconducting transition temperatures above 77.degree. K. (the boiling point of liquid nitrogen) are commonly found for these materials, and even higher transition temperatures are considered possible based upon recent revisions to existing theories explaining superconducting behavior. The economic advantage that these new superconductors could have over previously existing lower superconducting-transition-temperature superconductors is large enough that many new uses for superconductors now can be devised and present uses enormously improved. However, because these new mixed-oxide superconductors are brittle, ceramic-like materials, they do not lend themselves easily to fabrication in the form of high strength, wire-type geometries, a requirement for many important uses to which superconductors have been put in the past. These uses largely revolve about strong field magnets used in high energy physics, traffic engineering, etc.
One way of fabricating a brittle superconducting material in wire-like form is set forth in an article by K. Brennfleck et al. entitled "Chemical Vapor Deposition of Superconducting Niobium Carbonitride Films on Carbon Fibers" which was published in Proceedings of the 7th Conference on Chemical Vapor Deposition, Electrochemical Society (1979) at p. 300. This article describes depositing a niobium carbonitride layer directly onto a THORNEL.RTM. 400 multifilament yarn by chemical vapor deposition (CVD) to form a superconducting composite. However, the Brennfleck et al. composites employ a low thermal conductivity, more reactive carbon fiber and the structure shown in the photomicrographs accompanying the article a poor physical structure. Additional aspects of niobium carbonitride-carbon fiber based superconducting composites are taught in U.S. Pat. Nos. 4,299,861; 4,581,289; and 4,657,776. Recently, ultrahigh modulus, high thermal conductivity carbon fibers of low resistivity have become available which will perform most, if not all, the stabilization required for carbon fiber superconducting composite operation. See, for example, U.S. Pat. No. 4,005,183 Singer granted to Union Carbide. Thus, the need for the outermost copper coating used in the previous literature for stabilization is either reduced or eliminated resulting in simpler and more economical devices.
The usefulness of an intermediate carbide or oxide layer between a carbon fiber and a niobium carbonitride superconductor layer to improve adhesion of the superconductor is taught in U.S. Pat. No. 4,585,696. Such a layer depends upon its intermediate (to the fiber and superconductor) coefficient of expansion to achieve its adhesive effect.
The new mixed-oxide ceramic-type superconductors are different in physical properties than the Brennfleck et al. niobium carbonitride material and these differences lead to different considerations for fabricating the superconductor into wire-like form. For example, the niobium compound has a cubic crystal structure and its critical current and critical fields are isotropic, i.e., the same along each of its three crystallographic axes. The new 1-2-3 superconductors on the other hand show a much smaller critical current and critical field along the c crystallographic axis than along the a and b crystallographic axes. Thus, it may be important to align the a b planes of the 1-2-3 superconductor microcrystals as completely as possible parallel to the fiber axis for maximum effectiveness when made in a superconducting device.
Now it has been found that ceramic-type superconductors such as the recently discovered R.E., Ba, Cu oxide-type superconductors can be formed on low resistivity, high thermal conductivity, high strength, ultrahigh modulus carbon fibers in adhering layers by several different techniques to yield useful superconducting composites. Additionally, it is possible that at least some preferred orientation of the superconductor microcrystals on the fiber can be produced, which composites can be formed into strong, flexible conductors capable of exhibiting substantial critical currents and critical magnetic fields under superconducting conditions.